1. Field of the Invention
The present invention relates generally to the art of antifriction bearings and more particularly to the art of ball bearings. More specifically, the invention relates to preloaded, angular contact ball bearings for precision instruments such as the spin bearings of gyroscopic instruments, and to a ball and cage structure for stabilizing bearing torque and preload.
2. Description of the Prior Art
The accuracy and stability of a gyroscope, that is, its ability to maintain its spin axis fixed in inertial space, is dependent, among other factors, on the characteristics of the ball bearings which support the spinning rotor. It has been observed that spin ball bearings utilized in Reaction Wheel Assemblies (RWA's) and instrument gyros exhibit low level long term torque periodicity (on the order of 2 to 60 minutes). This torque variation is evidenced in very small but undesirable periodic wheel speed changes in the case of RWA's (resulting in spacecraft position changes) and drift variations in the case of gyros. The magnitude of these variations is very small, on the order of 5 percent or less of the mean torque at speeds to 6 KRPM.
While is is desired that all of the balls of a ball bearing set be identical in size and all perfectly spherical, it is recognized in the art that such perfection is normally unattainable. Also, those skilled in the art recognize that conventional ball cage designs leave much to be desired in terms of stability and mechanical as well as audible noise. Given these practical limitations, even the most carefully manufactured ball bearings will not provide ultimate antifriction support, particularly of the precision required for supporting a gyroscope rotor, since the slightest imperfection in the rotor shaft support bearings can result in a long term drift of the gyro. The foregoing variations in torque have been found to be due to gyroscopic precession of the individual balls of the ball set, resulting in variations in the ball diameter presented to the points of angular contact with the ball races and an associated variation in preload. There have been no known previous attempts to stabilize ball bearing torque by preventing ball precession. It is probable that the existence of this torque periodicity as well as the reason for it is not commonly known. For these reasons it is doubtful that prior related art exists.
It has been discovered by the present inventor that a major source of these variations has been traced to small preload variations and changing differential ball spin velocities caused by shifts of the balls' spin axes as the balls precess. This invention solves the problem of torque periodicity by preventing ball precession by combinations of mass inertia configuration control of the balls and mechanicl capture of the balls.